1. Field of the Invention
This invention relates to an optical recording medium of phase change type.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectivity by the crystallographic change is detected for reproduction of the information. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
The optical recording medium of phase change type utilizes difference in reflectivity between the crystalline and the noncrystalline state, and light absorption (Ac) of the recording layer in the region other than recording marks (in crystalline state) and the light absorption (Aa) of the recording layer in the recording marks (in noncrystalline state) are often different, and the Ac&lt;Aa is the condition generally found in such situation. Recording sensitivity and eraseability are thus different depending on whether the region overwritten is crystalline or noncrystalline, and consequently, the record marks of different length and width are formed by the overwriting to invite increase in the jitter often resulting in errors. When mark edge recording wherein the information is encoded in opposite edges of the record marks is adopted for increasing the recording density, variation in the length of the record marks has greater significance and such variation invites increased errors. In order to solve such a situation, the absorption should be adjusted to Ac=Aa, or Ac&gt;Aa in consideration of the latent heat, by regulating the thickness of the recording layer or the dielectric layers sandwiching the recording layer. In the medium of conventional structure, the adjustment to Ac.gtoreq.Aa results in reduced difference between the reflectivity (Rc) of the medium of the region other than the record marks and the reflectivity (Ra) of the medium in the record marks, and hence, in a reduced C/N.
In view of such situation, Japanese Patent Application Laid-Open No. 5(1993)-298747 proposes provision of a thin reflective layer of not more than 15 nm comprising Au or an alloy thereof on the recording layer with the intervening dielectric layer, and discloses .DELTA.A (=Ac-Aa) of 5% or higher, and .DELTA.R (=Rc-Ra) of 15% or higher.
Japanese Patent Application Laid-Open No. 8(1996)-124218 proposes an optical information recording medium comprising a substrate, a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer, a third dielectric layer, and a UV curing resin layer disposed in this order wherein Ac&gt;Aa, and an extremely thin metal film of high light transmission, Si or Ge is used for the reflective layer, and a dielectric material having a refractive index of higher than 1.5 is used for the third dielectric layer. Increase in Ac/Aa is achieved without detracting from the high (Rc-Ra) by providing the reflective layer of high light transmission and the third dielectric layer of high refractive index.
Provision of the reflective layer of high light transmission as described in the above-cited the patent publications is associated with the problems as described below. When a dual side optical recording medium is prepared by bonding two substrates each having a recording layer disposed thereon by means of an intervening adhesive layer, the adhesive layer deposited by roll coating, screen coating, or the like is associated with unevenness or mark in the coating direction, and such unevenness is converted by the optical pickup into noise, inviting increase of jitter and failure in track servo and focus servo. In the case of a single substrate optical recording medium, a resin protective layer is provided as a top coat on the side of the recording layer, and when the resin protective layer is formed by such process as spin coating, the noise caused by the uneven coating is the trouble.